Microporous polyolefin membranes are widely used in separators for lithium batteries, etc., electrolytic capacitor separators, various filters, etc. When the microporous polyolefin membranes are used as battery separators, their performance largely affects the performance, productivity and safety of batteries. Particularly lithium ion battery separators are required to have excellent mechanical properties and permeability, as well as shutdown property, a function of closing pores to stop a battery reaction at the time of abnormal heat generation, thereby preventing the heat generation, ignition and explosion of the battery, which can be caused by the short-circuiting of external circuits, overcharge, etc.; heat shrinkage resistance, a function of keeping a separator shape to avoid a direct reaction between a cathode material and an anode material even when becoming high temperatures, etc.
Recently gaining importance as separator characteristics are not only permeability, mechanical strength, heat shrinkage resistance and thermal properties (shutdown properties and meltdown properties), but also battery life properties such as cycle properties (properties concerning battery capacity when used repeatedly), and battery productivity such as electrolytic solution absorbability. A lithium ion battery electrode expands by the intrusion of lithium when charged, and shrinks by the departure of lithium when discharged, an expansion ratio when charged tending to become larger as recent increase in the capacity of batteries. Because a separator is compressed when the electrode expands, the separator is required to suffer only small permeability variation by compression to have excellent cycle properties. To that end, there are (i) a technology of providing a separator with a gradient structure comprising a coarse-structure layer having a relatively large average pore diameter, which undergoes large deformation with small air permeability change when compressed, and a dense-structure layer having a relatively small average pore diameter, which undergoes large air permeability change with small deformation when compressed, the coarse-structure layer absorbing the expansion of an electrode and holding permeability; and (ii) a technology of making the deformation of the entire separator small to prevent a pore structure from being broken. These technologies are properly selected depending on the properties of electrodes.
To improve the electrolytic solution absorbability, it is effective to provide a large pore size to the separator surface. Also, to prevent by-products generated by the repetition of charge/discharge cycles from clogging the separator, the separator is required to have a large pore size on the surface. However, to secure the mechanical strength, a dense layer is needed. Thus, to satisfy both requirements of high electrolytic solution absorbability and high mechanical strength, the separator is desired to have a coarse-structure layer having a relatively large average pore diameter on at least one surface, in addition to a dense-structure layer.
Liquid filters are desired to have higher filtering performance, and for this purpose, microporous membranes should have smaller pores. However, to avoid decrease in the filtering efficiency, the microporous membrane should not deteriorate liquid permeability. To meet both requirements of high filtering performance and high liquid permeability, the liquid filters desirably have the above gradient structure. Specifically, the balance of the filtering performance and the liquid permeability can be controlled by constituting the microporous membrane by a dense-structure layer as a support layer and a coarse-structure layer as a filtering layer, and adjusting the thickness ratio of the dense-structure layer to the coarse-structure layer.
A microporous polyolefin membrane, JP 2000-212323 A discloses a microporous polyolefin membrane different between the internal structure and the surface structure to have excellent pin puncture strength and porosity, which has an average pore size of 0.01 to 0.2 μm, at least one surface thereof having an average pore size of 0.5 to 2 μm. This microporous polyolefin membrane is produced by (i) melt-blending a polyolefin and a plasticizer to prepare a polyolefin solution, extruding and cooling the polyolefin solution to form a sheet, stretching the sheet, and then extracting the plasticizer from the stretched sheet to form a microporous membrane 1 having an average pore size of 0.5 to 2 μm on at least one surface, (ii) further stretching the microporous membrane 1 while heating to form a microporous membrane 2 having an average pore size of 0.01 μm or more, and (iii) laminating the microporous membranes 1 and 2.
JP 2003-105123 A discloses a microporous polyolefin membrane comprising polyethylene having a mass-average molecular weight (Mw) of 5×105 or more as an indispensable component, and having an average pore size change in a thickness direction, wherein at least one surface thereof is larger in average pore size than inside, or wherein one surface being larger in average pore size than the other surface, so that the microporous polyolefin membrane has excellent pin puncture strength, heat shrinkage resistance and permeability. This microporous polyolefin membrane is produced by (a) melt-blending a polyolefin comprising polyethylene having Mw of 5×105 or more as an indispensable component with a membrane-forming solvent, extruding the resultant melt blend through a die, cooling the extruded melt blend to provide a gel-like sheet, biaxially stretching the gel-like sheet with a temperature distribution in a thickness direction, removing the solvent from the stretched gel-like sheet, stretching the resultant membrane in at least one direction, and then heat-treating the membrane at a temperature in a range of the crystal dispersion temperature of the polyolefin or higher and lower than the melting point of the polyolefin to form a microporous membrane (i), (b) stretching the above gel-like sheet in at least one direction at a temperature lower than the crystal dispersion temperature of the polyolefin, and then stretching the gel-like sheet in at least one direction at a temperature in a range of the crystal dispersion temperature of the polyolefin or higher and lower than the melting point of the polyolefin, and further removing the solvent from the stretched membrane to form a microporous membrane (ii), and (c) laminating the microporous membranes (i) and (ii). In the microporous membranes of the above references, however, layers having different average pore sizes are formed under different stretching conditions, but not under different melt blend concentrations. Accordingly, they do not necessarily have well-balanced permeability, mechanical properties, heat shrinkage resistance, compression resistance, shutdown properties and meltdown properties.